Hybrid vehicles typically include a high voltage battery pack adapted to provide power to various components of the vehicles, including motors, transmissions, and electrical accessories. A battery pack may consist of various chemistries, such as lithium ion, nickel metal hydride, or nickel cadmium. The battery pack may also include multiple cells connected in series, parallel, or a combination thereof. Each cell may include a cathode, an anode, and an electrolyte. The electrolyte is a medium that allows the electrical charge to flow between the anode and the cathode. It is the electrochemical reactions that occur within the cell that allows a battery pack to serve as an energy source.
The performance of the battery packs may degrade overtime because of cell decomposition or abuse conditions, including but not limited to overcharging, internal shorting, external shorting, and over-discharge, among others. Gas build-up and subsequent increase in pressure within a cell may occur from the decomposition of the cells or the abuse conditions. The increase in pressure may occur relatively slowly due to decomposition of the cell over the cell's life. Or the increase in pressure may occur rapidly in response to a temperature increase in the cell. Depending on the battery type, cell venting may be characterized by the cell being pressured, discharge of gases from the cell, such as vaporized solvents, or propulsion of the electrolyte out of the cell. Some batteries may include vent ports to accommodate the pressure, gas release, or electrolyte being propelled out of the cells.